This invention relates in general to an improved wire rope and, more particularly, to a rope having a central fiber core comprised of aramid or other high strength synthetic elements.
Within the wire rope industry, there is a class of roping materials that are known by the term "elevator system ropes". These materials are used in a drive system as (1) hoisting ropes providing suspension of freight and passenger elevator cars and the vertical displacement of same by means of traction drive, (2) counterweight ropes used for suspension and vertical displacement of system counterweights and (3) compensator ropes which can be used in conjunction with 1 or 2 above.
In the U.S. elevator industry, standard elevator rope sizes range from 3/8" to over 3/4" (9.5 to 19.0 mm). Most of such ropes have a central core member comprised either of a monofilament polypropylene or natural fiber such as manila, sisal, or jute. Typically, such ropes have outer strands of various grades of steel in a 6 or 8 strand arrangement.
In addition, elevator hoisting ropes comprising an independent wire rope core are currently in use in Europe for large structures, albeit with a unit rope weight penalty approaching 30%.
The decreasing availability of natural fibers such as manila, jute, mauritius or sisal has led to a shift to synthetic fibers in attempts to provide an adequate core material. Widely used synthetic monofilaments such as the polyolefins or nylon, are not yet accepted as a core material by the elevator market due to possible hygroscopic character, low effective modulus and relatively low compression resistance. These factors result in higher stretch values and increased likelihood for strand to strand contact and earlier onset of fatigue.
The development of high strength synthetic materials, such as the polyamide and polyolefin families, having relatively high coefficients of elasticity along with lower weight compared to steel has resulted in attempts to hybridize or develop rope sections to take advantage of the benefits these fibers offer. The superior environmental exposure resistance, along with the precision available in the manufacture of monofilament yarns of specific denier, provides the rope manufacturer with the ability to hold closer tolerances with these synthetics versus natural fiber materials.
Past inventions have attempted to incorporate these materials in a multitude of applications, some of which are hybrid forms, using steel outer strands over a synthetic core as presented in U.S. Pat. Nos. 4,034,547, 4,050,230 and 4,176,705, and South African Pat. No. 86-2009. In these patents the cores of the ropes are said to be of parallel or minimal lay designs, with the cores made up of monofilament yarns, in attempts to maximize elastic modulus and associated tensile strength. The major drawback of this approach is that ropes of this type, when loaded, shift the majority of the load onto the central core, which yields in tensile before maximum load can be imparted to the surrounding steel strands.
The conservative design factor and sheave criteria imposed in elevator standards shifts the rope performance requirement from that of strictly strength over a minimal life to that of fatigue resistance, with expected lifetimes reaching 5 years or more. The rope is expected to maintain diameter to provide proper bedding in traction sheaves, with the outer steel strands being expected to provide a tractive interface between rope and sheave as well as enduring tensile loadings and bending stresses as the rope passes through the system. The fiber core must meet a separate set of parameters, maintaining its integrity and uniformity of diameter and density, while resisting decomposition or disintegration, in order to support the rope strands for the full lifecycle of the rope.